1. [common; from the ASCII mnemonic for 0000110] Acknowledge. Used
to register one's presence (compare mainstream Yo!).
An appropriate response to ping or
ENQ.

2. [from the comic strip Bloom County] An
exclamation of surprised disgust, esp. in “Ack pffft!”
Semi-humorous. Generally this sense is not spelled in caps (ACK) and is
distinguished by a following exclamation point.

3. Used to politely interrupt someone to tell them you understand
their point (see NAK). Thus, for example, you might
cut off an overly long explanation with “Ack. Ack. Ack. I get it
now”.

4. An affirmative. “Think we ought to ditch that damn NT
server for a Linux box?” “ACK!”

There is also a usage “ACK?” (from sense 1) meaning
“Are you there?”, often used in email when earlier mail has
produced no reply, or during a lull in talk mode to
see if the person has gone away (the standard humorous response is of
course NAK, i.e., “I'm not
here”).